A 20 year-old male with recurrent syncope and wide QRS tachycardia with LBBB type underwent radiofrequency catheter ablation. Symptoms were present for 8 years. Electrophysiologic study revealed an antidromic atrioventricular reentrant tachycardia using an atrioventricular Mahaim fiber which conducted only anterogradely and resulted in LBBB QRS morphology on preexcitation. Decremental conduction was also noticed during single atrial extrastimulation and rapid atrial pacing. The atrial insertion of the Mahaim fiber was at the lateral tricuspid annulus and the ventricular insertion was at the lateral right ventricle near the tricuspid annulus. Discrete Mahaim fiber potential was recorded during atrial pacing and during tachycardia. Mahaim fiber was successfully ablated at the atrial side of lateral tricuspid annulus where the discrete Mahaim fiber potential was recorded. No complications occurred. Tachycardia did not recur during a follow-up of 6 months.